Mount Prospect, Illinois 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

236
FXUS63 KLOT 122300
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
600 PM CDT Sun Jul 12 2026

.KEY MESSAGES...

- Hot and humid conditions are expected for much of the upcoming
  week; though somewhat cooler near Lake Michigan.

&&

.DISCUSSION...
Issued at 0600 PM CDT Sun Jul 12 2026

An already strong mid-upper level ridge of high pressure over the
Rockies will strengthen further as it builds eastward across the
northern Plains into the western Great Lakes/upper Mississippi
Valley through midweek. Most guidance has 500mb heights peaking
around 6000m over the upper Mississippi Valley. To put into
perspective just how impressive that is, in the nearly 80 year
long record of upper air soundings from the Minneapolis area,
the highest 500mb height ever record was 5980m.

This near-record strong upper ridge will set the stage for
several days of hot and likely humid conditions across much of
the region. The core of this heat dome does look to stay just
north of our area with the highest temp anomalies likewise
expected to remain just north of our area. While the most
intense heat should remain to our north, it still looks likely
to get quite hot locally, especially Tuesday and Wednesday when
high temps are likely to top out in the low to mid 90s across
our entire CWA.

There remains considerable uncertainty in the dewpoint forecast,
which will dictate just how high heat indices will get. Low level
trajectories will be pretty light with the origins of our air mass
solidly continental with no contributions from the Gulf. The
expected increase in dewpoints over the next few days will be
almost exclusively driven by evapotranspiration (ET), a process
which isn`t explicitly modeled, but rather handled by a
parameterization and then fed into the physical model. This
certainly opens the door to potential errors, but given we are
entering the climatological peak of ET, the model guidance that
depicts dewpoints increasing into the 70s by Tuesday and Wednesday
seem fairly plausible. Subsidence associated with the very strong
upper ridge should result in a fairly low inversion and likely
prevent deep mixing which could allow the more shallow low level
moisture to mix out. Ultimately, the NAM overall did a good job
with temps and dewpoints with the heat wave a couple of weeks ago,
so opted to lean heavily on that model for temps/dewpoints through
Wednesday. This looks to result in heat indices peaking in the
upper 90s to slightly over 100 degrees both Tuesday and Wednesday
afternoon.

By Thursday, the GFS is more aggressive with weakening of the
upper ridge and for a couple of runs now depicts a back door cold
front resulting in some meaningful break in the heat late in the
week. The ECMWF and GEM maintain a stronger upper ridge and do not
have a backdoor front on Thursday. Given the spread, the NBM
seems to offer up a reasonable compromise in solutions, but the
spread in potential high temps/peak indices for Thursday and even
Friday is fairly large.

Proximity of the surface ridge should result in weak low level
flow which should allow or lake breezes to form each of the next
several days. While southern Lake Michigan water temps are
currently running close to average, they should warm to solidly
above average in the coming days given the light winds, warmth,
and full sun during the day. The resultant lake breezes will
probably only cool temps slightly into the low/mid 80s, with
that more pronounced cooling likely confined to areas very near
the shore. The magnitude of cooling as well as the timing and
extent of inland penetration of the lake breeze will play a
pivotal role in just how hot it will get over the densely
populated urban corridor of Chicago. If the higher dewpoints do
materialize, then it is possible that portions of Cook County
farther inland from the lake could see heat indices peak near or
just over 100F Tuesday and Wednesday with the potential for
some late afternoon/early evening moderation of the heat
depending on the progress and strength of the lake breeze.

Unless dewpoints overperform even the most aggressively moist
models, then we will likely generally remain below headline
criteria for heat. The one exception could be Cook County away
from the lake which could reach its 3 day Extreme Heat Warning
criteria Tuesday through Thursday, assuming that the backdoor
front doesn`t bring more meaningful relief. Given the potential
for dewpoints to mix out, lake cooling to be more prominent, and
the possibility of a backdoor front on Thursday, the chances of
needing a heat headline for Cook County look to only be about
30-40% at this point.

Late this week and into next weekend medium range guidance is in
pretty good agreement in breaking down the upper ridge with more
of a northwest flow pattern setting up locally. This could result
in a more active pattern locally with chances for showers/storms
increasing late in the week into next weekend.

- Izzi

&&

.AVIATION /00Z TAFS THROUGH 06Z TUESDAY/...
Issued at 559 PM CDT Sun Jul 12 2026

Only Concern:

- Potential lake breeze wind shift to easterly at ORD late
  Monday afternoon

Quiet, VFR flying conditions are in store through the period.
Easterly winds early this evening will become light/VRB to calm
overnight and then light west-northwest after sunrise Monday.
A lake breeze will push inland, crossing MDW in the early
afternoon, but then slow on its approach to ORD. Plenty of model
guidance does not feature a wind shift, but given how light the
flow aloft is, still think an easterly shift is plausible enough
to maintain mention in the TAF. Main change was to push the wind
shift timing back a few hours. Confidence is on the lower side.

Castro

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...None.

&&

$$

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